Assessment ofpower swing blocking functions ofline ...iie.fing.edu.uy/publicaciones/2008/SFG08a/SFG08a.pdf · Assessment ofpower swing blocking functions ofline protective relays

Assessment ofpower swing blocking functionsof line protective relays for a near scenario of

the Uruguayan systemc. Sena, R. Franco, A. Giusto.

Abstract - During normal operation of an electrical powersystem, there will be some conditions like faults or otherdisturbances which may cause loss of synchronism betweenareas. When loss of synchronism takes place, asynchronous areasshould be separated to avoid equipment damaged or extensiveoutages.

This paper describes the assessment of the protection systemof a part of the Uruguayan power system.

The focus is on the incorporation of a big consumer withgeneration capacity. The involved added apparent powergeneration is about 10% of the maximum historical Uruguayanload.

The article describes a set of studies on the protective systemassociated to transmission lines feeding the new consumer.Particular attention is paid to blocking power swing functions ofthe involved relays.

Power system simulations and relay tests have beenperformed, to evaluate the performance of the relays underpower swing conditions. The foreknowledge makes possible toavoid undesired operation.

Index Terms - Out-of-step, Power swing, Power swingblocking, Power system stability, Power system transientstability.

I. INTRODUCTION

POWER systems are subjected to a broad number of smallor larger disturbances during normal operating conditions.

Small disturbances, like changes in loading conditions, arealways present. The power systems must remain stable tothese changing conditions and continue to operate within thedesired limits of voltage and frequency [1].

The transient stability of a power system is a complexphenomenon that involves its dynamic response, including theaction of the control and protective devices, in the face of adisturbance. Sudden events as the loss or application of largeblocks of load, line switching, generator disconnection, orfaults often originate a significant transient response that leadsthe system to another equilibrium point. Large systemdisturbance could cause large power flow swing and couldlead to unwanted relay operation and power blackouts.

PDT (Ministerio de Educaci6n y Cultura - Programa de DesarrolloTecnol6gico) does the financial support of the project "Short Term ScenariosStability Studies in the Uruguayan Electric System" of Facultad de Ingenieria- UdelaR. This paper presents one of its studies and results.

978-1-4244-2218-0/08/$25.00 ©2008 IEEE.

The Uruguayan network qualitatively consists of boththermal and hydro-electrical units facing a sustained growingpower demand and the addition of significant generators andindustrial consumers. It is strongly connected with theArgentinian system through a double link in 500kV, and withthe Brazilian system via a 70 MVA back-to-back converter.The voltages of the transmission networks are 500kV and150kV, and the voltages of the sub-transmission anddistribution networks are 60kV and 30kV.

The power system has the characteristic that majorgeneration comes from the hydro-electrical units that arelocated in the north and centre of the country. The majorconsumer centre is located in the south of the country with thethermal electrical units.

The Uruguayan power system is now facing theincorporation of new consumers with the possibility to operateas consumers or generators. These new consumers haveinstalled thermal electrical units for 140MW, consuming 90 to120MW, and injecting the surplus to the Uruguayan powersystem.

When long distance separates the load centre from thesupply, the power system weakening between the two areasmay lead to out-of-step conditions and cascading lines outagesbecause of distance relays operations.

Section II summarizes the relevant relationships betweenpower system stability and distance protection. Section III andSection IV describe the simulations and tests performed andits results. Finally the conclusions are presented in Section V.

II. DISTANCE PROTECTION AND POWER SYSTEM STABILITY

There are many technical books and papers describingpower system stability and its relation with distanceprotection. Following there is a brief review based on thedescription in [2]-[5].

Power oscillations are normal balanced events in powersystems. They may be the consequence of any disturbance inthe power system such as line switching, faults, load sheddingand generators tripping. During normal operation themagnitude of the oscillations are usually small and veryattenuated. But during abnormal operation the oscillations canbe larger.

The loss of synchronism between areas affects thetransmission lines relays. Distance relays elements may

Authorized licensed use limited to: Universidad de la Republica. Downloaded on December 16, 2008 at 11:35 from IEEE Xplore. Restrictions apply.

operate during a power swing, if the impedance locus entersthe distance operating characteristic.

A. Distance relays

Distance relays respond to positive-sequence quantities.The impedance measured by the distance relay during a powerswing depends on the phase angle separation 8 between thetwo equivalent system source voltages. During a power swing,zone 1 distance relay function and pilot functions will be thefunctions with more chances to operate. Fig. 1 shows theoperation of a distance relay when the impedance measuredgoes into the operating characteristic.

Zone 1

2

x

Fig. 3: Trajectory of the impedance measured during a power swing

C. Power swing Detection methods

The traditional and most common method used in powerswing detection is based on measuring the positive-sequenceimpedance and the transition time through a blockingimpedance area in the R-X diagram. The movement of theimpedance for short circuit faults is faster compared to themovement for a power swing. Fig. 4 shows two differentcharacteristics of a scheme used for power swing detection.

Fig. 1: Zone 1 Characteristic

B. Impedance measured during a power swing

The two machine system in Fig. 2 can be used to describethe performance of distance protection during power swings.The impedance seen by the relay at C during a power swingcan be determined by (1).

Outer Elem.ent

/ Inner Element

. .,. ~t~le swing

Distance Element

x

.- Unsta.bleswing

Fig. 2: Two machine system

During a power swing the angle 8 will vary. For stableswing, the angle 8 increases to a maximum value when thetrajectory shifts direction and 8 decreases to a minimum wherethe trajectory shifts direction again. This sequence repeatsuntil the power swing ends. For unstable swing, the trajectoryreaches 8=1800

•

Fig. 3 shows the trajectory of the impedance measured. Thetrajectory of the impedance measured by the distance relay isa straight line when lEAl = IEBI. For lEAl> IEBI and lEAl < IEBI,the trajectory is circular.

Fig. 4: Circular and quadrilateral power swing detection characteristic

A timer is started when the impedance measured enters theouter characteristic. If the measured impedance remainsbetween the inner and outer characteristic for the set timedelay, it is considered a power swing and the tripping of therelay is blocked during a certain time. But if that impedancecrosses the inner and outer characteristic in a time shorter thanthe set time delay, it is considered a short circuit and thetripping of the relay is allowed.

Other modern methods detect power swings based onpermanent measurements, and are applied alone or combined.They are possible because of numerical relaysimplementation.

Some of these methods consist on permanentmeasurements that in the R-X diagram evaluate traj ectoryspeed, trajectory monotony, trajectory coincidence betweenloops, trajectory in zones of steady state instability, electricalsystem centre estimation, etc.

III. POWER SYSTEM MODELING

This section describes the simulations and tests that havebeen performed, to study in the Uruguayan System thebehavior of some relays under power swing conditions. Theobjective was to obtain, by simulation, power swingconditions relative to the new generators incorporated to the


electric system.Time-domain simulations of the network have been carried

out by using ATP [7] and DSAT [8] programs, whilelaboratory tests have been done in relays using secondaryinjection test equipment.

Both numerical and experimental investigations of thepower swing phenomena were performed in order to study thebehavior of the line protections relays of the transmissionlines near the new consumer.

The technique chosen in this research was to simulate thepower system in great detail using ATP, with previoustransient stability studies in DSAT.

A number of different technologies and techniques areavailable to test a relay. One of them is to feed relays with theoutput of the ATP program (.pI4 file format) using secondaryinjection test equipment.

A. The power system under study

The grid configuration has both, thermal and hydroelectrical units in service. As it has been mentioned, thehydro-electrical units are located in the north of the country,while the thermal electrical units and the load centre are in thesouth. The new big industrial generator-consumer which islocated near Fray Bentos (FBE) 150 kV substation, in the westof the country. Fig. 5 presents the network disposition of theinvestigated part of the Uruguayan transmission powersystem.

The protection relays that are installed in the transmissionlines under study have quadrilateral power swing detectioncharacteristic. These relays are SIEMENS 7SA611 and7SD522, and use modern methods to detect power swings,evaluating trajectory speed, trajectory monotony, trajectorycoincidence between loops, and trajectory in zones of steadystate instability, with an outer quadrilateral characteristicelement for power swing detection covering the innerquadrilateral fault detection element and all distance zones(zone 1 to zone 5) [6]. Those relay's combined algorithmsproduce the "Power Swing" internal signal in order to blocktripping.

BOT BOTI5(1kV

I......

IlBBI5(1W

3

possible to carry out the first part of the power swing studiesin TSAT program, in order to define the contingencies tosimulate and analyze.

Faults in 150 kV lines near Fray Bentos were simulated inorder to obtain power swings for the new generator against therest of the system. Critical clearing times (CCT) werecalculated for each interesting fault.

Three phase faults are the worst cases, that determinate theshorter clearing critical times, so these faults were the firstones to be studied.

Then stable and unstable power swings near criticalclearing times were simulated and analyzed with the help ofthe impedance diagrams of the distance protections in thetransmission lines.

Sudden line openings were simulated, but they generate norelevant power swings.

Unexpected loss of generation and load shedding werestudied too, and some cases were decided to be studiedfurther.

After all these studies, a set of pairs: contingency protection line relays of interest, was selected to be simulatedfurther in ATP.

c. Simulation ofpower swings in the Uruguayan electricsystem with ATP.

The three phase power system has been modeled for thepurpose of generating currents and voltages at relayslocations. The signals were obtained by simulating the powersystem using the ATP. The power system model is heavilybased on a previous Uruguayan - Argentinian electricalnetwork model [9].

It was created and included the ATP model of the bigindustrial generator-consumer incorporated recently to theUruguayan power system (reason of this study). After that, itwas possible to simulate the studies of interest in order toperform the relay's tests.

The contingencies selected before in the TSAT studies andsimulated in ATP, were:a) three phase short-circuit over critical time, without

automatic reclosing, in FBE-SJA 150 kV line, near FBE,b) three phase short-circuit over critical time, without

automatic reclosing, in MER-YOU 150 kV line, nearMER,

c) sudden total load shedding in BOT,d) sudden loss of one of two generators in BOT.

In these simulations, voltage and current signals in selectedlines were generated and stored in pl4 format.

Fig. 5: Power system under study

B. Simulation ofpower swings in the Uruguayan electricsystem with DSA T.

The scenarios under study and their operational parametershad to be defined as part of the inputs to begin the studies.This was done by migrating to DSAT the original PSSIEmodel of the Uruguayan system including detailed excitationcontrol models.

With the help of DSAT database scenarios, it have been

IV. SIMULATION AND TEST RESULTS

The signals generated with ATP were injected to realdistance protections, equal or similar to the ones installed inthe selected points of the power system.

An OMICRON CMC256-6 test set for secondary voltageand current injection was used to perform dynamic transienttests of SIEMENS 7SD522 (87L/21) and 7SA611 (21)protective relays.


Results of the simulation of the four contingencies selectedare shown below.

In the short circuit contingencies, the simulated voltagesand currents consist of 500 ms pre-fault, then the fault that lastmore than critical clearing time, and post-fault up to 5 s oftotal simulation time.

In the load rejection contingency and the generatorrejection contingency, the simulated voltages and currentconsist of 1s pre-contingency, and 5 s of post-contingency.

The R-X plots were generated with SIGRA (SIEMENS)managing oscilography software. Each plot shows thecorresponding distance zones and the locus of seenimpedance, but do not show the outer quadrilateralcharacteristic of power swing detection that is at least 1 ghigher than distance zones.

A. Three-phase short-circuit in FBE-SJA 150 kV line, nearFBE, for over critical clearing time, cleared by both ends'relays without automatic reclosing, in FBE-SJA 150 kV line,near FBE.

4

Power swings were detected in post-fault, and relays settheir "Power Swing" internal flag on from 10 to 60 ms toblock tripping if protection picks-up. But in general laterpower swings are stable and do not even enter protectivezones as shown in Figs. 7 to 10, so relays do not pick-upduring post-fault, except BOT-»FBE relay in Fig. 10.

In Fig. 7 FBE-»MER relay picks-up in the reverse timedelayed zone and in the non-directional time-delayed zonewhen short-circuit happens.

Impedances

RlOtm(secondary)

Z4

Fig. 7: Impedance seen by FBE~MER 7SA611 protection.

In Fig. 8 MER-» FBE relay picks-up in the forward timedelayed zone when short-circuit happens.

Impedances

,,_:-- "+_,,P __I" j j 1 ,

t~~--,- I ~~_,__ ' \r~}- _ .:...

~

•a

•••

So....----

~

&.'-~ *''*' .,:s' -'it.. .....-::;...,...-..;.--- --• .,.~r,~·--~ --,

RlOhm(S8COndary)

Z4

Fig. 8: Impedance seen by MER~ FBE protection.

In Fig. 9 FBE-»BOT relay picks-up in the reverse timedelayed zone when short-circuit happens.

Impedances

F 2

~ 1

~ o+----------~~-----

! :

RlOtm(secondllY)

----Fig. 6: Example. Voltages and currents injected to MER~FBE protection.

In the beginning of post-fault power swings, the frequencymeasured by relays of interest is approximately 3.2 Hz.

The impedance loci seen by the relays in this contingencyare shown in Figs. 7 to 10.

Relays have seen the impedance entering their protectivezones (in the event of the fault) and leaving them (when thefault is cleared). In both events, the not so fast transition effectof relays' phasors calculation moving data window, is shownas not so fast loci movements in the R-X figures produced bySIGRA software too.

Fig. 9: Impedance seen by FBE~BOT 7SD522 protection.

In Fig. 10 BOT-» FBE relay picks-up in the forward timedelayed zone when short-circuit happens.

RlOhm(second&ry)

Fig. 10: Impedance seen by BOT~FBE 7SD522 protection.

Concluding, in this contingency relays' distance functionspick-up but never trip due to power swings, and relays' power


swing blocking function operates properly.

B. Case: three phase short-circuit in MER- YOU 150 kV line,near MER, for over critical clearing time, cleared by bothends' relays without automatic reclosing, in MER-YOU 150kV line, near MER.

~I.---

5

Impedances

Fig. 12: Impedance seen by FBE~SJA 7SA611 protection.

As an example, FBE~SJA relay's "Pickup" flag, and"Power Swing" flag in post-fault power swing in the left ofFig. 12 are shown in Fig. 13.

. -

.----..-..-

... -I

"d· ',j.

UN IVoltages

I1\.,..-

-=j- ---... . --.-- --B

~-

~]---

Fig. 11: Example. Voltages and currents injected to FBE~ SJA protection.

In the beginning of post-fault power swings, the frequencymeasured by relays of interest is approximately from 2.4 Hz to3.6 Hz.

The impedance loci seen by relays of interest in thiscontingency are shown in Figs. 12 to 16.

Relays have seen as power swings both: impedances duringfault and impedances after fault is clear. Transitions ofimpedance due to the effect of protection's moving datawindow are not so notorious, because of the slowness ofpower swings during fault and post-fault.

When power swings were detected in post-fault, relays settheir "Power Swing" internal flag on from 130 to 150 ms forblock tripping if protection picks-up. But later power swingsare stable and do not even enter protective zones as shown inFigs. 12 to 16, so relays do not pick-up during post-fault.

But power swings were also detected by SJA~FBE andBOT~FBE relays during fault, as explained below for Figs.14 and 16. Relay's "Power Swing" signal is also set on duringmost of the fault, when locus enters and leaves protectivezones.

In Fig. 12 FBE~SJA relay picks-up the reverse timedelayed zone and the non-directional time-delayed zone onlyat the end of short-circuit.

Fig. 13: Currents and voltages recorded by FBE~SJA 7SA611 protection;power swing and pickup relay's signals are shown (fault and post-faultrecord).

In Fig. 14 SJA~FBE relay picks-up the forward timedelayed zone and the non-directional time-delayed zone onlyat the end of short-circuit. Relay's "Power Swing" signal is seton during most of the fault.

RlOhm(secondary)

Z4

Fig. 14: Impedance seen by SJA~FBE protection.

In Fig. 15 FBE~BOT relay picks-up the reverse timedelayed zone only at the end of short-circuit.

Impedances

f ,.I ..-t--------'---------,,---------+-------------

!x '.

RlOhm(S8COndlWy)

Z4

Fig. 15: Impedance seen by FBE~BOT 7SD522 protection.

In Fig. 16 BOT~FBE relay picks-up forward time-delayedzone. Relay's "Power Swing" signal is set on during most of


6

the fault.

~ ,.i ..+-----.----------+---------------

~ '0

Fig. 16: Impedance seen by BOT~FBE 7SD522 protection.

~;".u-iIVII

'I"S.

.... -.u--......~.... -

~

-u-~'i

"'1

: .. "

Concluding, in this contingency, relays' distance functionspick-up but never trip due to power swings, and relays' powerswing blocking function operates properly.

C. Case: sudden total load shedding in BOT.

~I.-

-=~g::at,,~) ---=.SlI::......~j _

Fig. 18: Example. Voltages and currents injected to BOT~FBE protection.

Neither FBE~BOT nor BOT~FBE 7SD522 protectionspick-up. No oscilography register was generated.

\WO~J - - \~ - --

Fig. 17: Example. Voltages and currents injected to BOT~FBE protection.

VI. ACKNOWLEDGMENT

The authors gratefully acknowledge the contributions andsupport of:

Partners of the Stability Studies Group of IIE-FINGUdelaR, specially Michel Artenstein and Pablo Monzon,

Graciela Calzolari and Claudio Saldana, for the model builtin ATP of the previous power system,

Protection Section of UTE, especially Julian Malcon, JuanZorrilla and more other partners.

VII. REFERENCES

V. CONCLUSIONS

This paper describes the method, studies and tests toperform in distance protections in order to know its behaviorunder power swing conditions.

The new incorporation of a big industrial generatorconsumer located near Fray Bentos 150 kV substation to theUruguayan electrical power system, with generation capacityof 140MVA (near 10% of the maximum historical load), addto the system new power swings where new generator areinvolved.

Simulations and tests under different system conditionshave shown that in order to maintain the integrity of the powersystem, the line protection relays operate in a properly way,detecting power swing and blocking trip.

This study shows that the distance protections located nearthe new consumer would be performed correctly under powerswing conditions, blocking the relays avoiding them tooperate and lead to major outages.

:; ~ ~

1'""-1....·"~

1IC'''SII--~''''1 --

} ! ,----- --- - -- - - --......................_................................._...

8:"lSiIlo-~~] --8C'l!DI:~ _

.... UI'-

~-

.....[';",;10 " ;

:-

~.L_·__'

--

Neither FBE~BOT nor BOT~FBE 7SD522 protectionspick-up. No oscilography register was generated.

D. Case: sudden loss ofone oftwo generators in BOT.

.. -~

..-----.-

.--

..-

.._.........-....

I'- ,-'-'- --;- .,. ..... "," .~ ".......----...............~----------

[1] P. Kundur, Power System Stability and Control, The EPR! PowerSystem Engineering Series, MC Graw Hill Inc, 1994.

[2] D. Tziovaras, D. Hou, "Out-Of-Step Protection Fundamentals andAdvancements", 30th Annual Western Protective Relay Conference,October 21-23,2003, Spokane Washington.

[3] "Power Swing and Out-of-Step Considerations on Transmission Lines",IEEE PSCR WG D6.

[4] G. Benmouyal, D. Tziovaras, D. Hou, "Zero-Setting Power-SwingBlocking Protection", 3 j1h Annual Western Protective Relay Conference,October 19-21,2004, Spokane Washington.


[5] J. Berdy, "Application of Out-aI-Step Blocking and Tripping Relays",GER-3180.

[6] SIEMENS 7SA6 and 7SD5 manuals.[7] ATP (ATP-EMTP Alternative Transients Program), software, theory

book and rule book manuals.[8] DSAT (DSATools, Dynamic Security Assessment Software, Powertech

Labs Inc), software and manuals.[9] G. Calzolari - C. Saldana, "Perdida de Paso Polar, 5ta. Unidad Central

Batfle ", July 2004.

VIII. BIOGRAPHIES

Celia Sena was born in Salto, Uruguay, in 1970.She received her degree in Electrical Engineering from the Universidad de

laRepublica, Uruguay, in 1997.She joined the Administraci6n Nacional de Usinas y Transmisiones

Electricas (UTE) in 1992, where she held the position of engineering in theSystem Protection Engineering section. Her interests include digital relaymodeling, power system protection and power system transients.

She is pursuing the master degree in electrical engineering.Since 2007. she has been with the Instituto de Inegenieria Electrica.She is member of the IEEE.

Ricardo Franco was born in Montevideo, Uruguay in 1962.He graduated in Electrical Engineering from Universidad de la Republica

(UdelaR), Uruguay in 1994. He is pursuing the master degree in ElectricalEngineering.

He works in the Protection Section of Transmission (and for Generationtoo) of UTE (Uruguayan national electricity generation, transmission anddistribution public enterprise) since 1989.

He is with the Instituto de Ingenieria Electrica, Facultad de Ingenieria,Universidad de la Republica (IIE-FING-UdelaR) since 2007.

IEEE Member since 1995.

Alvaro Giusto was born in Montevideo (Uruguay) in 1965.He graduated in Electrical Engineering (1992) from the Universidad de la

Republica, Montevideo (Uruguay) and obtained the M.SC. Degree (1995)from the Universidade Federal de Santa Catarina, Florian6polis (Brazil). He iscurrently pursuing the Ph.D. degree. He is with the Instituto de IngenieriaEIectrica, Facultad de Ingenieria, Universidad de la Republica since 1990.

He have been working as independent consultant in automation andcontrol since 1998. His research interests are power system stability, controltheory and control applications.

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Assessment ofpower swing blocking functions ofline ...iie.fing.edu.uy/publicaciones/2008/SFG08a/SFG08a.pdf · Assessment ofpower swing blocking functions ofline protective relays